If she is at a good weight and that works then just be aware of the sugars in different hays and when it is cut. There are times they can be on pasture and times they shouldn't. Some wear grazing muzzles others are essentially dry lotted year round. Really depends on the extent of the founder. There are pelleted feeds that work if they need the extra calories and fats are your best source for providing more if something above that is needed though those usually have a relatively high fat content. Pics of your mare? There are several here with experience in your neck of the woods. I am sure they will be along to offer advice.